**Core Concept**
Infectious mononucleosis, also known as glandular fever, is a viral illness caused by the Epstein-Barr virus (EBV). It primarily affects adolescents and young adults, but can also occur in children. The condition is characterized by fever, sore throat, lymphadenopathy, and lymphocytosis.

**Why the Correct Answer is Right**
The presentation of acute lymphadenopathy, fever, and 20% lymphocytosis in an 8-year-old child is suggestive of infectious mononucleosis. The lymphocytosis in this condition is typically due to an increase in atypical lymphocytes, which are a hallmark of EBV infection. These atypical lymphocytes are a result of the virus's ability to infect and transform B lymphocytes, leading to their proliferation and accumulation in the peripheral blood.

**Why Each Wrong Option is Incorrect**
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**Clinical Pearl / High-Yield Fact**
Infectious mononucleosis is often diagnosed based on clinical presentation and laboratory findings, including a positive monospot test (heterophile antibody test) and the presence of atypical lymphocytes. It's essential to note that EBV infection can be asymptomatic in some cases, but when symptoms do occur, they can be severe and prolonged.

**Correct Answer:**
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